1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Recently, along with a downsizing trend of electronic appliances such as cellular phones and portable personal computers and an increase in demand for them, there is a mounting demand for higher performance in secondary batteries used as a power source of these electronic appliances. To meet such a demand, nonaqueous electrolyte batteries using a material, such as a carbon material, capable of intercalating and deintercalating lithium ions as a negative electrode material have been developed, and are used widely as a power source for portable electronic appliances. The nonaqueous electrolyte secondary battery is, unlike the conventional battery, characterized by light weight and high electromotive force, and its excellent performance has been noticed. In particular, the portable personal computer has varied functions, including Web browsing, electronic mail exchange, and video viewing. Accordingly, the battery used as a power source is required to have not only larger capacity, but also higher output, that is, excellent large current discharge characteristic and pulse discharge characteristic.
For higher output of a secondary battery, it is needed to decrease the output resistance (internal resistance) of the battery. It is hence extremely important to lower the resistance of electrodes and battery constituent members.
For this purpose, various secondary batteries have been proposed, including a cylindrical lithium ion secondary battery or square lithium ion secondary battery having plural current collecting leads attached to electrodes thereof, as disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 11-317218 and 11-339758. Such secondary batteries are improved in current collection efficiency and lowered in output resistance, so that enhancement of the output characteristic is achieved.
Jpn. Pat. Appln. KOKAI Publication No. 2002-110254 discloses enhancement of the output characteristic of a lithium ion secondary battery by reducing the thickness of the electrodes. In these electrodes, in particular, by reducing the thickness of the positive electrode, diffusion of lithium ions in the electrodes is accelerated, and a lithium ion secondary battery of high output can be obtained.
However, when plural current collecting leads are connected to the electrodes as disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 11-317218 and 11-339758, the assembly procedure is complicated, and consequently, productivity of batteries is lowered.
In addition, when the battery is designed by reducing the thickness of the electrodes as in Jpn. Pat. Appln. KOKAI Publication No. 2002-110254, the occupying rate of current collectors in electrodes is larger, and the quantity of reaction substances such as active materials of electrodes is inevitably decreased. This is significantly disadvantageous in terms of advance of larger capacity of the secondary battery. Therefore, to achieve both larger capacity and higher output of the secondary battery, it is further required to lower the resistance of battery constituent members without reducing the thickness of electrodes more than necessary.
On the other hand, the lithium ion secondary battery has other problems, that is, overcurrent flows in the battery in the event of an abnormality such as overcharge or short-circuiting, a nonaqueous electrolysis solution is decomposed, and a decomposition reaction of the electrolysis solution causes heat generation to raise the battery temperature, or liquid leaks or causes a rupture. As a countermeasure, the lithium ion secondary battery incorporates, as one of battery constituent members, a ring-shaped PTC element for limiting the flow of current due to elevation of resistance when the battery temperature rises by overcharge or the like. The PTC element has a structure in which an element main body showing a sharp resistance increase along with temperature elevation caused by overcurrent or the like is disposed between a pair of electrodes.
By forming the PTC element in a ring shape, a sufficient gas passage can be provided. That is, a rupture plate which is broken by elevation of internal pressure due to gas generation is separately assembled in an external can to the inner side from the PTC element so as to be connected to the PTC element. The rupture plate breaks to release gas when the battery internal pressure is raised due to heat generation or another abnormality. At this time, by forming the PTC element in a ring shape, a gas passage is maintained in the hollow space, so that gas can be smoothly discharged and released outside.
In the PTC element, however, since its material configuration and shape, especially shape aspects, function as relatively large resistance components, elevation of output of the secondary battery may be hindered.